Email subscriber engagement is increasingly important as a determinant of inbox placement and positioning. For email marketers, the challenge is to identify drivers of positive engagement, and applying that knowledge to create campaigns that their subscribers consider informational, educational, or amusing. Success is being measured by important new metrics such as “retrieved from spam”, “deleted after reading”, and “marked as not spam” as these metrics increase, so will program performance.

Subject line testing has always been a simple yet highly effective way of identifying positive behavioural drivers. The principle is easy – prior to the main broadcast, create two or more test cells, create different subject lines for a common email creative, identify which one performs best (opens, clicks, conversions), then apply the winning subject line to the remaining subscribers. I’ve regularly seen this approach driving positive uplifts in campaign responsiveness of 33% or more, when benchmarked against the worst-performing alternative.

However, even subject line testing requires a degree of organization that isn’t always available to email marketers, who are notoriously time poor. I’ve often found myself wishing for a tool that would answer those magical questions (“What’s the best subject line length?”, “Which words can’t I use?”, “Are special characters a bad idea?”) without running a load of tests first. Now, to my unalloyed glee, I have such a tool at my fingertips, and I spent a few happy hours last week using it to test (and deconstruct!) some conventional email wisdom.

To provide some context – I have recently worked with email marketing programs in the “Restaurants” sector, so I pulled a 30 day snapshot from our reporting network, comprising 826 discrete campaigns, from a range of casual dining vendors. The key metrics that I evaluated against were “Read Rates” (a highly positive subscriber engagement metric) and “Spam Complaint Rates” (highly negative).

The first test focused on campaign size. I categorized each broadcast as small, medium, or large, with a hypothesis that “smaller” equated with more targeted audiences while “larger” corresponded with more generic audiences. This was spot-on – read rates for smaller campaigns were higher than the sector average, but declined as campaign size increased. Spam complaints for smaller campaigns were below the sector average, but increased as campaign size increased.

I then used the subject lines to categorise emails by message type. A well-understood element of email best practice is to avoid bombarding subscribers with a steady stream of offers, and identifying other touch points to create a more varied subscriber experience. Again, the numbers backed up the rationale – on average, welcome emails generated read rates that were almost 3 times higher than the benchmark, with thank you emails, birthday emails, and anniversary emails all generally twice as effective. Birthday emails also carry an important lesson about relevance – complaint rates for this message type are twice the benchmark, suggesting that for emails where date of send is crucial to success, getting it wrong will be punished.
 
Using ostensibly “spam trigger” words (“Special”, “Offer”, “Free”) all delivered read rates 25% to 50% higher than the sector average. However, there was also a corresponding trend of increased spam complaint rates. Clearly, the use of these words generates something of a love-hate response from subscribers, and marketers need to evaluate whether the upside outweighs the downside for their programs.

Subject line length also demonstrated some clear variations in subscriber responsiveness. I categorised them as: short< = 25 characters; medium <= 50 characters; long > 50 characters. Short subject lines performed best, with a 10% uplift in read rates against the benchmark, while medium subject lines under-performed by a similar measure. Interestingly, responsiveness then started swinging back toward long subject lines. While a short, punchy subject line that doesn’t get truncated would seem the preferred approach, longer subject lines that carry more detail about the email’s contents are also effective.

Lastly, I evaluated the effect of commonly-used special characters on subscriber responsiveness. Top-level results were that using financial amounts is good (read rates up 10%), as is exclamation marks (read rates up 30%), but subscribers clearly don’t like being too challenged – read rates went down by 20% when the subject line posed a question!

All of these findings are a bit of fun, and I wouldn’t want readers to start quoting these numbers as the new industry gold standard. However, there is a serious point to be made – email subject lines represent the point of departure for any journey that leads toward improved subscriber engagement. Email marketing is highly competitive, and smart practitioners are investing time and resource to identify the approaches to which their subscribers will respond most positively. And, as with all competitions, to the victor go the spoils – in the form of improved deliverability, greater subscriber responsiveness, and increased campaign ROI.

Register now for this virtual summit and learn all about the latest trends and best practices in email marketing without leaving your desk!

And it won’t cost you a penny/eurocent/dollarcent/… 

The DMA is proud to be a sponsor of this, the very first edition of the International Email Marketing Summit.

Not only will you be inspired by the latest tactics that work but you’ll also take away a list of action items you can implement immediately.

Featured speakers

• Dela Quist, Alchemy Worx
• Dave Chaffey, Smart Insights
• Tamara Gielen, Plan to Engage
• Denise Cox, Newsweaver
• Riaz Kanani, Alchemy Worx
• Kath Pay, Plan to Engage
• Arianna Galante, ContactLab
• Tom Bailey, eCircle
• James Bunting, Communicator

What’s on the agenda?

• Beyond just selling: engaging with your subscribers
• 7 reasons why your subscribers don’t respond
• Tips & tricks for designing emails for a mobile audience
• Inactive Subscribers: Prospects or Problem?
• Creating a successful content strategy for email marketing: 8 Easy Steps
• and lots more…

In this issue of Infobox, there’s a lot more to integrating social media and email than just dropping in a few “share to social” icons. Dave Chaffey of SmartInsights, comes up with some inspiring examples of channel integration from the likes of uSwitch and Mothercare.

Dave Chaffey is among the speakers at a free, virtual International email marketing summit, which takes place on 16 May and is sponsored by the DMA.

Elsewhere in this issue, Simon Hill from Extravision analyses email deliverability across the globe and I reveal why I look forward to receiving e-newsletters from The Innocent Group in Campaigns we like.

Kath Pay, editor, Infobox
Co-Founder, Plan to Engage

PS – want articles like these delivered to your inbox twice a month? Then sign up to Infobox here!

It’ll surprise no-one to hear that smartphones and mobile devices are changing the way we receive email. Research suggests 63% of women and 71% of men access email services on their mobile device.

But it’s important to remember this isn’t just a platform used on the bus or in shops. There’s just as much chance you’ll find someone surfing through emails on their mobile while at home on the sofa.

It is perhaps the most ubiquitous way of consuming email in the modern day. So it’s important to take it seriously.

Whether that means making sure emails render properly on mobile devices or ensuring a smooth transition when recipients click through to a mobile landing page, mobile integration can’t be just an afterthought – it needs to be discussed and considered at a strategic level.

Use what you already know

One piece of good news in considering mobile email is that you don’t have to invest in new technologies to get ahead of the game- you’ve got them already. Many email service providers will be able to tell you the exact devices your recipients are using to read your emails and help you focus your attention on certain screen sizes or platforms.

Once you have this baseline understanding, the other important signal to look for is whether this audience is growing and how fast. The opportunity once you have this group segmented is to send them different emails optimised for the platform and test open rates and reactions.

A big part of the potential success here will depend on your business – what works for a retailer may not be so effective if you are a massive B2B services house. But there’s only one way to find out and get ahead of the game.

Responsive design is the future of email

Even better than a separate version though is a relatively old trick, stolen from the world of website development, called responsive design. This allows you to intelligently juggle two style sheets within the same email, but it’s not often that I see this used in all the emails I receive.

It means you can have a more balanced layout specific to the screen size your recipients are using, without obscuring any important calls to action or rendering emails disproportionately.

The best of both worlds.

The last mile

What often lets recipients down is the user experience when they click through to the landing page.

Of course, responsive design could also provide a solution here on your mobile landing page conundrum. Many email marketers get everything right when it comes to the message itself, only to be let down when mobile recipients click through and find a landing page that is impossible to view without some radical scrolling.

It has been argued that perhaps for certain customer databases, mobile opens are not that high or that the rendering of emails on mobile email clients isn’t that bad on templates around 600 pixels wide. However, I would go the extra mile and take a serious look at media queries and responsive design if your mobile open rates are high. This is where it will pay off the most.

It is important to remember that every campaign is a process and that equal attention should be paid to all parts of the recipients’ user journey. Make the most of info you already have – especially when it comes to mobile.

Simplicity, clarify and accessibility

With mobile email marketing, simplicity, clarity and accessibility are always the most important considerations. Smartphone screens are small, the connection can be flaky and a recipient’s time is short. If that recipient is to be worth anything to you, you should consider their time as money!

Mobile presents a world of opportunity and it’s vitally important to tap into this but, at the same time, it is crucial to keep on top of changing trends. Innovations in mobile technology are being so rapidly adopted that some recommendations are out of date as soon as they are published.

Register for this event

Attend this breakfast briefing at the trendy Hospital Club to hear the latest findings of the DMA/Return Path email deliverability white paper.

You will learn about the changing trends in deliverability that email marketers need to be aware of, plus hear practical recommendations and best practices on how to deal with the new challenges in getting your emails delivered into the inbox.

Gondola Holdings (incorporating Pizza Express, Ask and Zizzi) will take you through how they improved their email deliverability with impressive results.

Why should you attend?

• Hear about the changing trends in deliverability that you need to be aware of
• Learn practical tips that can help you get more emails into the inbox, increase your ROI and take your campaigns from good to extraordinary
• Discover best practices on how to deal with the new challenges

Who should attend?

Marketing and CRM professionals responsible for email marketing, eCommerce and ensuring maximum ROI is generated from email campaigns.

Agenda

8.30am Registration and refreshments

9.00am Welcome from the chair, Chris Combemale, Executive Director, DMA

9.10am Research findings, Guy Hanson, Director, E-Marketing Response Consulting Services EMEA, Return Path, Chairman, Legal Data & Best Practice Hub, DMA Email Marketing Council

9.40am Improving email deliverability, John Sullivan, Director Group IT & Marketing Systems, Gondola Holdings (incorporating Pizza Express, Ask, Zizzi)

10.10am Panel Discussion Guy Hanson,Director, E-Marketing Response Consulting Services EMEA, Return Path, Chris Combemale, Executive Director, DMA, John Sullivan,Director Group IT & Marketing Systems, Gondola Holdings (incorporating Pizza Express, Ask, Zizzi)

When
April 24th, 2012 8:30 AM until 10:30 AM

Contact
Phone: 020 7291 3349
Email: megan.hawkins@dma.org.uk

Event Fee(s)
DMA Member (inc VAT) £ 0.00
Non Member (inc VAT) £ 36.00

Location
The Hospital Club
24 Endell Street
London
WC2H 9HQ
United Kingdom

Register for this event

I think most marketers have heard about email authentication and the use of Domain Keys Identified Mail (DKIM) and the Sender Policy Framework (SPF). If your using an email service provider then its likely that they will handle the authentication for you and you don’t need to worry. But why do emails need to be authenticated ?

Authentication wasn’t created to reduce spam, althought indirectly it does help, but it was designed to reduce the amount of phishing emails and emails with fake sending addresses. It is easy for anyone to send emails pretending to be from a particular domain and without authentication you have no way of knowing if the sender is valid or not. Authentication works using DNS (Domain Name Service) records. You can think of DNS as a giant phone book for the internet. To check if an email is authenticated the receiving email server will look in the phone book to verify details about the sender. Only the owner of the sending domain has the ability to change entries in the phone book (DNS records) which makes the authentication process secure.

Unfortunatley this does not entirely stop phishing emails. A spammer can easily purchase a domain very similar to the one he is trying to copy and then setup the authentication correctly. The emails will pass authentication and you may think nothing is wrong unless you look closely at the domain being used.

In January 2012 the new DMARC (Domain-based Message Authentication, Reporting & Conformance) standard was released and I have to admit I was a little confused to start with. People talked about it strengthening DKIM and SPF but didn’t understand how ? Then suddenly the penny dropped. DMARC is in essence a two way feedback loop for authentication.

Until now, the SPF and DKIM standards have lacked a communication loop between the sender and receiver. A sender has no way to tell the receiving ISP that emails from this domain are authenticated and any that aren’t should be quarantined or rejected. Conversely the receiving ISP also has no way of telling the sender that the emails it is receiving are passing or failing authentication. DMARC provides this communication mechanism and closes the loop between the sender who is authenticating messages and the receiver who is trying to interpret these records. DMARC, like DKIM and SPF is another DNS record that is added by the domain owner or administrator.

DMARC doesn’t change anything in the way the two authentication processes work but brings them together in one standard. To fail DMARC you need to fail both SPF and DKIM authentication.  The feedback from the receiving ISP’s is in the form of an aggregate report. Currently we have only ever received reports from Gmail but hopefully Yahoo and others will follow soon. The report is sent daily in a zipped tar file and is in xml format.  You can import the report into Excel so you can read it like a spreadsheet or use a website such as www.dmarcian.com to format the results. What is staggering about the report is the number of emails that are being sent from other mail servers pretending to be from domains under your control.  You can enter the IP address of mail servers where the authentication is failing into www.projecthoneypot.org and immediately see if the address is a known spammer.

Authentication doesn’t help your emails find its way through the mine field of spam filters and smart inboxes but without it it is likely your emails will be rejected at the first hurdle. DMARC gives the sender a way of checking if emails are being authenticated correctly and what action the reciever should take if authentication fails.

You can learn more about DMARC at the website www.dmarc.org

Developing a good email marketing strategy can be a daunting task. To help you get some perspective, here are 3 key stages to keep you on track.

Develop a customer centric communications strategy.

I know this can be a bit of an overused statement, but to make the email channel work in the modern environment of priority inboxes’ etc it is vital. Focusing on the needs and motivations of the customer as they would relate to your brand is a great place to start. If you are going to be talking to the customer and expect them to engage, purchase or become loyal as a response, you’ve got to say the right things. This can’t be done at a campaign tactical level, when the heats on to get more sales to hit target; it needs to be part of an overriding communications strategy. This strategy will set out more than just how many promotional emails need to be sent to achieve revenue objectives. To develop this email communication strategy, these are some of the key elements.

• Understand how your customers perceive the brand and its products or services.
• Research the motivations and needs that engagement with your brand satisfies.
• Research the strengths that define your brand equity.
• Define your customer lifecycle and set business rules to identify where each customer sits.

Focus the strategy on increasing LifetTime Value

Once you have got a clear idea of your customer and the stages the customer goes through in their engagement with you (from discovery to defection), you can start to plan. One key objective of any email communications strategy should be to increase revenue by increasing customer lifetime value. Now, don’t think this is purely a retention statement, it equally counts for acquisition too.

Acquisition

If you’re going to be focusing on lifetime value, it will have an impact on which sources you target for acquisition. Customers coming from sources that provide a low lifetime value customer should be avoided, or the price you pay for acquiring the prospect should reflect their future value. In the case of an email address, they are not all worth the same, so the first task would be to identify sources of prospect email addresses that will provide good future customer value. A good place to start is to look at any results you have from past activity, and look at the overall sales achieved over time, from those customers.  The problem with email is that it is a cheap marketing medium that can be abused with little (apparent) cost implication. Good email prospect data, costs far more than poor quality prospect data, but can be far cheaper in the long term, as it produces good long term results.

Retention

Without the understanding of the customer (and you’ll only get this from the research suggested above) you won’t be able to sell to the customer what they want, how they want it. You’ll only be able to sell your product or service how you perceive it. Customer knowledge also allows you to tailor communications for each stage of the customer lifecycle. This will make your communications more relevant, more effective and more likely to meet business objectives. The strategy should be one that makes every marketing communication be seen as a positive experience by the customer, not a negative “interference” experience.   Ensure you do this by following these key rules:

• Define the commercial objectives for each stage of the customer lifecycle.
• Develop a customer communication plan that reflects the customer research and meets business objectives.
• Ensure research and testing is part of the strategy, to promote future development.

Make it part of a multi touch, multi channel strategy

In a connected world, where people are hooked onto the grid in multiple ways, touch points come via multiple channels. Just to take one example of a device, the smartphone can deliver a marketing message via email, web, social and SMS simultaneously. Studies have been suggesting recently that someone is likely to be watching the telly or walking round a store while access their phone, so the potential for cross media confusion abounds. Add this to multimedia spamming potential, and it makes integrated marketing communications essential for each channel’s success. Email has an important role to play in future direct marketing, with its unique strengths, it can only be effective as part of an overall cross channel strategy. Complimenting other channel activity, email often drives an uplift on other channels as well.

Taking a strategic approach to the email channel can bring lots more opportunity to the party, ultimately allowing customer knowledge to drive content, timing and targeting; nudging that little bit closer to true one to one marketing.

The Email Marketing Council’s Legal Data & Best Practice (LD&BP) hub has been reviewing the current email marketing best practices document over the past few months, and the publication of a revised version is imminent.

One of the things that the review process has identified is a need for more detailed guidance in certain key areas of the email marketing customer life cycle. For this reason, a number of supporting white papers have been produced, which can be found in the “Toolkit” section of the DMA’s website (www.dma.org.uk/toolkit), where they are available for download free to Members.

Here’s a quick summary of what has been produced to date:

Deliverability: Aimed at email program owners who have realised that their broadcasts are experiencing delivery problems, and are trying to identify why this may be the case. Looking at key factors such as sender reputation, spam filtering, blacklist operators, the document provides common-sense guidance on how to deal with them, including 10 easy-to-follow steps to improve your email deliverability.

Creative: Good creative is still an important determinant of a successful email campaign, and is sometimes the only connection a subscriber has with your brand. This document demonstrates that email creative is not a dark art requiring witchcraft and technical know-how! Rather, in non-technical language, it provides some easy-to-implement recommendations that will quickly optimise the performance of your email campaigns.

Data Analysis & Segmentation: Sets out a simple process to help email marketers start segmenting their data, and analysing their results. It defines five key areas to focus on, including: setting objectives; finding the right data; choosing the right segments; different segmentation models, and; effective use of segmentation. It also examines the best methods and approaches to implementing segmentation, as well as how best to interpret the results.

Split Testing: Provides email marketers with the basic capabilities that they will need to run split-testing activity. It looks firstly at the fundamentals that need to be in place to run a split testing program, and then examines ten prime opportunities where split testing can be introduced into any email marketing program to identify the optimal approach to maximise campaign response rates.

Triggered Campaigns: Delivering timely and relevant email messages, using trigger-based email marketing, plays an important part of email best practice. By analysing subscriber behaviour and identifying meaningful changes and/or events, organisations can communicate with their customers at a point when they are most likely to be receptive. This strengthens customer relationships by making them feel valued, and it is not unusual for trigger-based emails to attract high open rates as a result.

In addition to the documents that have been described above, there are also three new white papers whose publication is imminent:

• Using 3^rd Party Data For List Rental & Lead Generation
• A Layman’s Guide to Email Marketing Law
•  Email Lifecycle Marketing

And there are a further two which are scheduled for arrival during Q1 of the New Year:

• Organic List Growth
• Measurement & Reporting

The production of these documents is a collaborative process and the Email Marketing Council, as the representative body of the much larger interest group, is constantly feeding in new ideas about key issues which email marketers would like to have expert guidelines for. Hopefully, the documents described in this article are servicing this need, but it would be great to have direct feedback on whether they are useful, and what the email marketing community would like to see produced next. If you have any feedback for us, then drop a line to email@dma.org.uk , or online via LinkedIn, Facebook or Twitter.

Guy Hanson Chairs the The Email Marketing Council’s Legal Data & Best Practice (LD&BP) hub. He is Director, Response Consulting for Return Path.

Deliverability remains one of the most discussed challenges in the field of email marketing, so I think it rightly deserves its place as the most written about and talked about area of the business for beginners and experts alike. As inboxes evolve and their sophistication continues to increase, new approaches to email marketing programmes are needed to ensure that legitimate opt-in email is not only received – but anticipated.

The Evolution of Email Deliverability:

1. The Dark Ages of Email Marketing. In the early days of email deliverability was a free-for-all. Email messages from personal contacts, brands and spammers flowed freely to inboxes.
2. The Vigilante Years. Soon thereafter blacklists were formed to keep a record of notoriously bad senders, and those lists which are still in use today are often a form of self-policing by the Internet community.
3. Reputation 1.0. The next evolution in deliverability was based on reputation – more specifically IP address reputation. ISPs maintain a list of abuse complaints, hard bounces and other unsavoury characteristics of sloppy senders to penalise those individuals and companies whose practices prompt cries of help from recipients.
4. The Intelligent Inbox. With a few hops, skips and jumps in between we currently find ourselves in a world where meeting those previous bare minimum requirements may get your message into the inbox, but it will likely not get your message viewed, read, or acted upon. In this world:

a. Inboxes are being prioritised intelligently to separate bulk email from more personal, bi-directional interactions. (See Gmail Priority Inbox)
b. Recipient behaviour and interaction with messages are helping inbox providers determine message placement and overall delivery.
c. New tools in today’s inbox make purging messages and banishing non-relevant senders and brands to the Junk box one-click easy. (See Hotmail Sweep)

3 tips can help improve your deliverability in the world of the intelligent inbox:

1. Acknowledge that engagement is the key to the kingdom. ISPs and inbox providers often vary in the approach they take to determine message delivery, but nearly all are beginning to focus on one consistent measure of whether a sender is reputable or not – and that is ENGAGEMENT. In the world of the intelligent inbox, consumer engagement with your email marketing messages is critical to continued email deliverability. Recipient opens, clicks and forwards are becoming as much a predictor of relevance and authenticity as unsubscribes and complaints are to abuse.

Focus on creating messages that drive engagement and activity and you’ll be well on your way to improved deliverability.

2. Meticulously monitor your online reputation. Spammers burn through IP addresses as they plague consumers. And whilst that practice of provisioning a new IP address and opening the floodgates used to fool the system – it can no longer. Reputation is a hard-earned and constantly-monitored reflection of your email marketing habits. And, increasingly, it is being tied not just to IP address but to your domain and the infrastructure through which you send your messages. Monitor your reputation relentlessly; Have your email provider prepare regular reports for you to help you monitor the reputation of your IPs and sending domains; Or, use third-party services to assist. Most importantly, respond quickly to any fluctuations in your online reputation.

Your online reputation is as important as your credit score. Monitor it continuously and respond to any fluctuations promptly.

3. Authentication must be at the core of your efforts. Sounds geeky, but it is a foundational element of deliverability. Either your company or your email service provider should have your sending domains properly configured for a range of authentication services currently in use, including: Sender Policy Framework (SPF), SenderID, DomainKeys and DKIM.

Authentication technologies are similar to your passport or biometric scans – they help prove to ISPs that your company and your email messages are who they say they are. Do yourself a favour today and ask your technical team or your email service provider to verify which authentication schemes you are currently provisioned for. And, if any of the four boxes above are left unchecked, take quick action to get that authentication in place.

One of the contentions that surround email marketing at the moment is the issue of when you retire an email address. Leading up to Christmas, when the heat is on, ambitious sales targets tempt even cautious marketers to push out the boat and send to everyone. If an email list is causing deliverability issues, it is quite common for a bit of a clean up to be suggested. It’s not a “stab in the dark” strategy, because when used correctly it can lead to a net increase in response and revenue.

However, you cannot ignore, when retired email addresses are mailed, they often produce some revenue. This almost flies in the face of the no response/retirement strategy, but in reality, some fine tuning is in order to squeeze all the value from your list.

To deal with this issue properly, you will certainly need response (sales) data for your customers, and need to know which email addresses the data relates too. In most instances the full picture of your list can only be achieved through wider knowledge of the customer.

All too often, the most responsive customers are the ones who have been opening and clicking your emails recently. But it’s also important to segment those who are no longer interested, from those that have disengaged from your emails due to a higher contact frequency than their needs require.

The first stage of the solution should be test the differing frequency of those people who haven’t opened or clicked for a while. Although a 6 month open/click window might be fine for some businesses, it might not suit those businesses with a longer sales cycle or a wider range of buying frequency. In these instances, sending mailings for twelve months or even longer might be better, but proper testing should help you decide when a customer is signalling defection.

If you have transactional data, you can use the principles of RFM (Recency, Frequency and Monetary value) to build up a model which predicts your most responsive customers. In an ideal world you could marry up the purchase RFM data alongside the online engagement data, to see the point where Recency for online engagement (opens/clicks/visits) signals a lapsed customer.

Using email response data, we create two segments, those that are recently engaged, and those that are not (don’t throw any away yet!). The engaged segment can carry on receiving the main campaign emails at the normal frequency. The less engaged segment now gets a rest (for about three to four times the normal frequency of you campaign emails). So if you generally send weekly, rest this segment for a month.

What we are trying to do is identify a segment within the email database that has stopped responding to emails due to a mailing frequency that is too high for them. By responding to the users behaviour, you are able to make changes to the email frequency of this group.

If people from this lower frequency segment, respond, it is important that they don’t go straight back into the main campaign mailing frequency, but give them more of a rest between mailings.

What we are trying to do is to start down the road of mailing people at a frequency that suits them, keeping them engaged and encouraging them to buy more. Managing frequency is the easiest way to respond to behaviour (or lack of it) but if you have more resource, you could try content too. One of the other top reasons why people stop opening emails, is that the emails are no longer relevant to them. The difficultly with content relevance, is that it relies on a deeper customer knowledge, or web behaviour data.

Unfortunately there will be those email addresses in the list that despite your best efforts will never be responsive again. So, at some point you will have to bite the bullet and let these addressees go. It is important to accept that the damage that is done to the whole email programme (in the shape of poor inbox deliverability and reduction in response) will outweigh any extra revenue gained by mailing these inactive email addresses.